1. Field of the Invention
The present invention relates to a sheet conveying apparatus which nips a sheet by a pair of conveying rollers and conveys the sheet, and to an image forming apparatus having the sheet conveying apparatus.
2. Description of the Related Art
A conventional image forming apparatus such as a copying machine, a printer and a multifunction machine includes a sheet conveying apparatus which nips a sheet using a pair of conveying rollers and conveys the sheet as a system which conveys a sheet. An electrophotographic system is widely used in image forming apparatuses The image forming apparatus of the electrophotographic system conveys a sheet after toner is transferred to the sheet to a fixing portion, applies heat and pressure to the sheet, and the toner image transferred to the sheet is fixed. Since heat and pressure is applied to a sheet which passed through the fixing portion, a shrinkage difference is generated on the front and the back of the sheet due to a difference in a moisture amount on the front and the back of the sheet or a difference in a toner amount on the front and the back of the sheet, and the sheet is curled in some cases.
Users require various functions from the image forming apparatuses, and in two-sided image formation, it is required to form an image on the front and the back of the sheet without misregistration. However, due to heat, the outside shape of a sheet which once passed through the fixing portion is changed and thus, in order to form an image without misregistration, it is necessary to precisely measure the outside shape of a sheet which passed through the fixing portion, and this is reflected to an image forming position when an image is formed.
FIG. 10 illustrates a conventional image forming apparatus having a sheet conveying apparatus provided downstream of a fixing portion.
As illustrated in FIG. 10, the sheet conveying apparatus includes a sensor portion 82 which measures a length of a sheet in the conveying direction so as to measure the length of the sheet after it passes through the fixing portion. The sheet conveying apparatus includes a pair of conveying rollers comprising a drive-side blast roller 80 and a follower-side rubber roller 81. The blast roller 80 is formed by subjecting a metal roller surface to blast processing. By conveying a sheet under high conveying force between the blast roller 80 subjected to processing to increase the friction coefficient and the rubber roller 81, slip is suppressed and conveying operation of a sheet is stabilized. Therefore, a sensor portion 82 detects a front end position and a rear end position of a sheet conveyed by the rollers 80 and 81, but since the rollers 80 and 81 prevent the sheet from slipping, it is possible to precisely measure a length of the sheet in the conveying direction.
According to the pair of conveying rollers using the blast roller 80 having high surface friction coefficient, if a sheet having a curled front end is conveyed and abuts against the blast roller 80, there is an adverse possibility that the front end of the sheet is caught and the front end is crushed, bent or ripped. Hence, it is proposed to provide a guide member which smoothly guides a front end of a sheet to a nip portion between the blast roller 80 and the rubber roller 81. This technique is disclosed in Japanese Patent Application Laid-open No. 2003-128306.
FIG. 11 illustrates one example of a conveying roller mechanism provided with a conventional guide member. As illustrated in FIG. 11, the conveying roller mechanism, includes a pair of conveying rollers 71 and 72 having a friction force required for transmitting a conveying force to a sheet S, and a ring member 70 having an outer diameter larger than that of the conveying roller 71 and having an outer peripheral surface of low friction coefficient. A plurality of conveying rollers 71 are disposed in the axial direction such that the conveying roller 72 and the ring member 70 alternately come into contact with the conveying rollers 71 in the axial direction.
With this structure, the ring member 70 functions as the guide member, a front end of a sheet is guided along a peripheral surface of the ring member 70, and the front end of the sheet can reliably be guided to a nip portion (pressure contact portion) between the pair of conveying rollers 71 and 72. Therefore, since the front end of the sheet does not hit the conveying roller 71 having high friction coefficient, it is possible to prevent the front end of the sheet from being crushed, folded or ripped.
However, the ring member 70 illustrated in FIG. 11 is in contact with the conveying roller 72 having high friction coefficient. Since the friction coefficient of the ring member 70 is lower than those of the pair of conveying rollers 71 and 72, a conveying force for nipping and conveying the sheet by the ring member 70 and the conveying roller 72 is smaller than a conveying force for nipping and conveying the sheet by the pair of conveying rollers 71 and 72. Thus, locations having different conveying forces are generated in the axial direction of the pair of conveying rollers, and there is an adverse possibility that the sheet is fed on the skew or creased.
A peripheral surface of the ring member 70 is shaved by the conveying roller 72 having high friction coefficient at the time of contact, and since the peripheral surface of the ring member 70 is shaved, the friction resistance of the peripheral surface is increased or the outer diameter of the ring member 70 is reduced. If the friction resistance of the peripheral surface of the ring member 70 is increased or the outer diameter is reduced, the guiding function of the ring member 70 is deteriorated. That is, there is a possibility that a front end of a sheet can not reliably be guided to the nip portion between the rollers 71 and 72, the front end of the sheet is caught on the peripheral surface of the ring member 70, and the front end of the sheet is folded, bent or ripped.